Quote:
Originally Posted by Kevin Sevcik
Another more complicated option is to run your shooter at less than the top speed of your motors. The reason it takes longer to spin up is because you're spinning at the top speed of the motor. At that speed, the motor produces zero net torque, and a pretty small amount of torque as it approaches that speed. Less torque means less acceleration, means longer spin up time.
So if you could set things up so that you're only running at, say, 75% of top speed, then your wheel would recover much more quickly. Plus, you'd have a consistent speed for the wheel no matter what your battery level was, since the top speed depends on the voltage you're running at.
The downside is that this is a little complicated to do. You can only do this through closed-loop control of the wheel/motor speed. So you'd need an encoder to measure the wheel or motor speed, and you'd need to set up and tune PID control for the motor speed.
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Quoted For Truth
I wanted to stress the section that I bolded, because the definition of "top speed" in this context is tricky. It isn't the top speed of the motor at 12V, it is the top speed of the motor at whatever effective voltage you are applying. That is to say that if you drive at a constant 50%, you will have exactly the same spin up issue as you are having at 100%.
Prove this to yourself by measuring the shooter speed vs time at a couple of values. As you approach adjusted free speed, the effective torque approaches zero. Measure how long it takes to get back to "close enough".
If you want to avoid this asymptotically long spin up, you have to have some sort of feedback loop. Kevin suggested a PID, and I agree with him.